1. Field of the Invention
The present invention relates to a method of compensating for tilt and/or defocus and an apparatus therefor, and more particularly, to a method of compensating for tilt and/or defocus by controlling the power and/or time required for recording according to the amount of tilt and/or defocus of an optical recording medium, and an apparatus therefor.
2. Description of the Related Art
When tilt or defocus occurs, or tilt and defocus occur at the same time in an optical disc which requires high density recording using a blue light laser, the effect of the tilt and/or defocus increases in comparison to the effect on a low density recording, which uses a red light laser. Therefore, a method of compensating for the effect is required.
When both an objective lens with a large numerical aperture (NA) and a blue light laser using a 400 nm short wavelength is used instead of using an existing red light laser (650 nm wavelength) in order to obtain a higher density, a system is affected as shown in Table 1, as follows. The factor which most affects recording is a reduction in the margin according to the increase in tilt and a shallowing of the focal depth.
TABLE 1NANANAEffect in NA 0.6ItemEffect0.60.650.85NA 0.85Spot diameter (relativeλ/NA10.930.70Capacity doubledsize)Focal depth (relativeλ/NA210.850.50Servo controldepth)bandwidthdoubledDisc tilt (relativeλ/tNA310.790.35Strict disc tiltmargin amount)allowanceDisc thickness changeλ/NA410.730.25Strict thickness(relative allowance)allowance in discmanufacturing
In addition, tolerances for radial tilt and tangential tilt in a current digital versatile disc-random access memory (DVD-RAM) are 0.7° and 0.3°, respectively. The basic characteristics of a disc must be met while remaining within these tolerances. Thus, for example, power, such as write power and erase power, must be maintained at a level which is sufficient to obtain the write characteristics defined in a disc specification.
However, when a blue light laser using a short wavelength (400 nm) is used to meet the increasing demand for high density recording, the effect of tilt becomes greater. That is, when a higher NA is used in order to obtain the same substrate thickness and high density, the value of coma aberration becomes much greater. Equation 1 expresses the coma aberration as follows:                               Comma          ⁢                                           ⁢          Aberration                =                              (                                                            n                                      2                    -                    1                                                                    2                  ⁢                                      n                    3                                                              *              d              *                                                NA                  3                                wavelength                                      )                    *                      (                          tilt              *                              π                180                                      )                                              (        1        )            where, n is the refractivity of a substrate, d is the thickness of the substrate, and NA is the numerical aperture of an objective lens.
FIG. 1 illustrates coma aberration in three dimensions according to wavelength and NA, when the thickness of a substrate is 0.6 mm, the refractivity of a substrate is 1.5, and tilt is 0.5°, using the equation 1. The figure shows that coma aberration increases as wavelength becomes shorter and numerical aperture becomes greater.
FIG. 2 illustrates the changes in beam peak intensity with respect to tilt. According to FIG. 2, as tilt increases, the recording beam peak intensity decreases at a wavelength of 400 nm more rapidly than at a wavelength of 650 nm. If recording is performed under this condition, the desired length and width of a recording mark cannot be recorded. As the NA increases, beam peak intensity decreases even at the same wavelength of 400 nm and 0.6 mm substrate thickness (t).
FIG. 3 illustrates changes in beam spot size with respect to tilt the beam spot size normalized by the beam size when tilt is 0° (Beam widthtilt/beam widthtilt=0). The figure shows that as tilt increases, spot size increases more at a wavelength of 400 nm than at a wavelength of 650 nm, and, as NA increases, spot size increases even at the same wavelength of 400 nm.
FIG. 4 illustrates changes in the maximum temperature-to-write power ratio (Tmax) with respect to tilt, with Tmax normalized by Tmax when tilt of 0° (Tmaxtilt/Tmaxtilt=0). The figure shows that as tilt increases, Tmax decreases more rapidly at a wavelength of 400 nm than at a wavelength of 650 nm, and, as write power (Pw) increases, Tmax decreases even at the same wavelength of 400 nm, while at a wavelength of 650 nm, Tmax is insensitive to changes in write power (Pw).
In addition, as the luminance effect of a short wavelength laser diode decreases when the power emitted therefrom changes according to the temperature change, the laser diode must emit luminance stably in order to read information recorded on a disc without error and to increase the reliability of an optical disc system.
FIG. 5 illustrates changes in Tmax with respect to tilt with Tmax normalized by Tmax for a tilt of 0° (Tmaxtilt/Tmaxtilt=0). As in FIG. 4, FIG. 5 shows that as tilt increases, Tmax decreases more rapidly at a wavelength of 400 nm than at a wavelength of 650 nm. In addition, as write time (Tw) increases, Tmax decreases even at the same wavelength of 400 nm, while at a wavelength of 650 nm, Tmax is insensitive to changes in (Tw).
Accordingly, since beam intensity decreases rapidly and the beam spot size increases at a wavelength of 400 nm with respect to tilt, the desired length and width of a recording mark cannot be obtained when recording, and therefore, power density ultimately decreases. In addition, when recording is performed on a disc that requires high density, a 400-nm wavelength laser beam is used and tilt compensation is required since the required temperature for forming an amorphous mark decreases rapidly with respect to tilt as shown in FIGS. 4 and 5.
One known method of compensating for tilt extends the tilt margin by, during disc manufacture, thinning the substrate thickness, which is currently 0.6 mm. However, since having a substrate thickness less than 0.6 mm causes problems in manufacturing and in disc characteristics, tilt compensation cannot be performed simply by manufacturing a substrate thinner than 0.6 mm. In addition, as the focal depth of the incidence beam becomes shallower, the defocus margin becomes smaller, which causes recording problems since recording is sensitive to even a small degree of defocus. This will now be explained referring to FIGS. 6 and 7, showing beam intensity and spot size, respectively, with respect to defocus in red wavelength and blue wavelength.
FIG. 6 illustrates changes in beam peak intensity with respect to defocus, and with the beam peak intensity normalized by beam peak intensity when defocus is 0°. As defocus increases, incidence beam intensity decreases more rapidly at a wavelength of 400 nm and 0.65 NA than at a wavelength of 650 nm and 0.6 NA. When recording is performed in this condition, the desired length and width of a recording mark cannot be recorded. In addition, as NA increases, beam intensity decreases even in the same wavelength.
FIG. 7 illustrates changes in beam spot size with respect to defocus, with spot size normalized by spot size when defocus is 0. As defocus increases, spot size increases more at a wavelength of 400 nm than at a wavelength of 650 nm. As NA increases, spot size increases even for the same wavelength.
Therefore, like the effect of tilt, defocus affects both the peak intensity and the spot size such that normal recording cannot be performed. In addition, simultaneous occurrence of defocus and tilt is a more serious problem. The beam shape, peak intensity, and spot size in the simultaneous occurrence of defocus and tilt are shown in FIGS. 8 and 9.
FIG. 8 illustrates changes in beam profile when both defocus and tilt occur at the same time. Curve 1 shows a normal-state beam shape; curve 2 shows the beam shape when defocus is 0.25 μm; curve 3 shows the beam shape when defocus is 0.5 μm; curve 4 shows the beam shape when tilt is 0.5°; curve 5 shows the beam shape when tilt is 0.5° and defocus is 0.25 μm; and curve 6 shows the beam shape when tilt is 0.5° and defocus is 0.5 μm.
FIG. 9 illustrates changes in beam spot size and peak power intensity when defocus and tilt occur at the same time. The abscissa shows a normal case 1, a case 2 where defocus is 0.25 μm, a case 3 where defocus is 0.5 μm, a case 4 where tilt is 0.5°, a case 5 where tilt is 0.5° and defocus is 0.25 μm, and a case 6 where tilt is 0.5° and defocus is 0.5 μm. The ordinate shows both the normalized value of peak intensity when tilt and defocus occur for peak intensity in a normal state, and the normalized value of spot size when tilt and defocus occur for spot size in a normal state. As defocus and/or tilt increase, peak power intensity decreases, and spot size increases. Peak power intensity decreases more, and spot size increases more, when tilt and defocus occur at the same time than when they occur individually.
As shown in FIGS. 8 and 9, when tilt and defocus occur at the same time, the effect is more serious than when tilt or defocus occurs singly. Accordingly, there is a need to compensate for tilt and/or defocus while performing high density recording.